The expansion of the areas of application of optical communication systems in recent years has brought with it a demand for smaller and cheaper optical modules. In particular, in subscriber optical communication systems, i.e., systems that use optical fiber for connecting users' homes and the stations of communication providers (also called “optical access systems” and “FTTH (Fiber To The Home”), wavelength-division multiplex communication is desired for realizing two-way optical communication through the use of two wavelengths in one optical fiber. As a result, the realization of a smaller and cheaper wavelength-division multiplex optical transmitter and receiver module required for such a communication system has become a crucial issue. Given these circumstances, optical waveguide modules are being investigated and put into practical use as constructions that enable smaller size and lower cost.
Explanation next regards prior-art examples of optical waveguide modules.
FIG. 1 shows the configuration of the first prior-art example of an optical waveguide module (disclosed in JP-A-2001-133666 and JP-A-2001-305365). Wavelength filter plate 102 that includes filter film 103 and light-shielding film 104 is secured by resin 106 to the end surface, which is close to the light-receiving element 107, of optical waveguide substrate 100 on which optical waveguide 101 is formed. Filter film 103 passes light of the wavelength which can be received by light-receiving element 107 and reflects light of other wavelengths. Light-shielding film 104 of wavelength filter plate 102 includes aperture 105 for passing the light which can be received by light-receiving element 107. Wavelength filter plate 102 is positioned with respect to optical waveguide substrate 100 such that aperture 105 is aligned with the optical axis of optical waveguide 101. This optical waveguide substrate 100 is secured to ceramic substrate 111 by solder 112. Light-receiving element 107 is bonded to light-receiving element carrier 109 by solder 110. Light-receiving element carrier 109 is bonded to ceramic substrate 111 by solder 113. In this optical waveguide module, the relative positional accuracy of the optical axis of light-receiving element 107 and the optical axis of optical waveguide 101 depends on the positional accuracy of light-receiving region 108 of light-receiving element 107 with respect to the contour of light-receiving element carrier 109 and the relative positional accuracy of light-receiving element carrier 109 and optical waveguide substrate 100 that are bonded to ceramic substrate 111.
A light-emitting device (not shown) is mounted on optical waveguide substrate 100. The light-emitting device emits light of wavelengths different from the wavelength which can be received by light-receiving element 107 and this light is propagated through optical waveguide 101. Nearly all of the light that is emitted from the light-emitting device and propagated through optical waveguide 101 is reflected by filter film 103 of wavelength filter plate 102, whereby unnecessary light that is incident on light-receiving element 107 can be reduced to a small amount. Light-shielding film 104 further cuts off the small amount of unnecessary light that is produced when leaked light emitted from the light-emitting device but not incident on optical waveguide 101 is propagated through optical waveguide substrate 100 and then passed through filter film 103, whereby the propagation of the unnecessary light to light-receiving element 107 can be prevented.
FIG. 2 shows the structure of the second example of the prior art of an optical waveguide module (disclosed in JP-A-H10-54917). Wavelength filter plate 123 is inserted midway in optical waveguide 121 that is formed on optical waveguide substrate 120. Wavelength filter plate 123 passes light of wavelength λ1 which can be received by light-receiving element 125 and emitted by light-emitting device 126, and reflects light of other wavelengths. Of the light of the plurality of wavelengths that is propagated through common port 122, only light of wavelength λ1 is passed through wavelength filter plate 123, and light of other wavelengths is reflected by wavelength filter plate 123 and directed to reflection port 124. Optical waveguide 121 is split into two after wavelength filter plate 123 to realize optical coupling with each of light-receiving element 125 and light-emitting device 126. Light-receiving element 125 and light-emitting device 126 are mounted on optical waveguide substrate 120 such that their optical axes are aligned with each of optical axes of two split optical waveguides 121. Leaked light that is emitted from light-emitting device 126 but that is not incident on optical waveguide 121 is propagated through optical waveguide 121 but cut off in the portions of light-shielding grooves 127, whereby the propagation of the leaked light from light-emitting device 126 to reflection port 124 can be prevented.
However, the above-described optical waveguide modules of the prior art have the following problems.
As previously described, in the first example of the prior art, the relative positional accuracy of the optical axis of light-receiving element 107 and the optical axis of optical waveguide 101 depends on the positional accuracy of light-receiving region 108 of light-receiving element 107 with respect to the outer shape of light-receiving element carrier 109 and the positional accuracy of light-receiving element carrier 109 and optical waveguide substrate 100 secured to ceramic substrate 111. Wavelength filter plate 102 must be secured accurately such that the position of aperture 105 is aligned with the light which is emitted from optical waveguide 101 through filter film 103 so that the light is not blocked. An optical waveguide module that is used in wavelength-division multiplex transmission requires accurate assembly such that divergence of the optical axis is kept small (error on the order of ±5 μm to ±10 μm). The configuration shown in FIG. 1, for example, requires three high-accuracy assembly steps (a step for attaching light-receiving element 107 to light-receiving element carrier 109, a step for securing optical waveguide substrate 100 and light-receiving element carrier 109 to ceramic substrate 111, and a step for attaching wavelength filter plate 102 to optical waveguide substrate 100), and therefore necessitates expensive high-accuracy assembly devices for this assembly. Two wiring formation steps (a step for forming electrical wiring between light-receiving element 107 and light-receiving element carrier 109, and a step for forming electrical wiring between light-receiving element carrier 109 and ceramic substrate 111) are further required. The fabrication of the configuration of the first prior-art example therefore requires many high-accuracy assembly steps, and these requirements not only increase fabrication costs but also impede improvements in yield. In addition, the need to realize high-accuracy assembly requires the use of a ceramic having excellent mechanical strength and a low level of heat distortion as the material of light-receiving element carrier 109 and ceramic substrate 111, and further requires high dimensional accuracy in which error is suppressed to approximately ±1 μm, further contributing to high fabrication costs.
The second example of the prior art requires the use of edge-incidence type light-receiving element 125 on which light is incident from the end surface. Edge-incidence type light-receiving element 125 involves higher costs than a conventional main surface-incidence type light-receiving element on which light is incident from main surface, and there are currently extremely few types of products that can be used. Furthermore, light-receiving element 125 must be mounted such that the error between the optical axis of optical waveguide 121 and the optical axis of light-receiving element 125 is on the order of ±1 μm, and this process requires an extremely expensive high-accuracy mounting device. In addition, for some types of optical waveguide module, the location at which light-emitting device 126 is mounted and the location at which light-receiving element 125 is mounted must each be separately provided on expensive optical waveguide substrate 120. In this case, the quantity of optical waveguide substrates 120 that can be fabricated from a single substrate is reduced, again resulting in higher costs.
In either of the first prior-art example and second prior-art example, determination of whether optical coupling between optical waveguides 100 and 121 and light-receiving elements 107 and 125 is adequate (whether the desired performance will be achieved) cannot be carried out until the assembly steps and wiring forming steps are completed. As a result, the problem arises that a typical active alignment method cannot be adopted in the step of aligning the optical axes when assembling the optical module. In other words, a method cannot be employed in which current is allowed to flow to the light-receiving element while light is emitted from the optical waveguide to the light-receiving element, the optical coupling efficiency between the light-receiving element and optical waveguide are monitored while the relative positions are adjusted such that the two components achieve the best positional relation, following which the two components are secured.